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3SL Reference - Needs in Systems Engineering

Concept and Purpose

Needs are expressions of outcomes to be achieved by new or existing products. The term objective is often used in business analysis to mean something achieved by following one or more steps. In a systems engineering context, objectives and needs can be considered equivalent. A goal is more intangible, a situation which an organisation seeks to achieve by completing a series of objectives, that is, by satisfying a set of needs.

There are multiple sets of needs:

  • A set generated internally in your business
  • A set from each request from each customer

We propose that these are not requirements as they are not in the language normally used for requirements, particularly for statements from customers. Customers will state their wishes in their terms, and not as desired changes in your products, and not as requirements for new products that you might produce.

Content and Creation

Needs are not written in the language of your products, nor in the language of the technologies used in your products. Rather, they will be written in the language of the customers’ products and business, or (for internal needs), written in the language of business or commercial objectives.

Examples of internal needs are:

  • “Reduce the number of types of bought-in component, to increase order volumes and purchase discounts.”
  • “Reduce software maintenance by standardising drivers across all products, disabling features where needed.”
  • “Achieve compliance with standard IEC 12345.”

Needs will be created by:

  • Capturing from documents or spreadsheets
  • Structured interviews, normally only for clarifications

Structure and Organisation

Needs are hierarchical. As their main characteristic is the group that produces them, we propose a hierarchy of internal needs, and one hierarchy for each customer. Further, since needs are the basis for projects, we propose a sub-hierarchy of needs for each project. Hence the overall structure of needs will be:

Overall Structure of Needs

If a project has needs from several groups, such as Project P12 above, it will have several needs hierarchies, all linked from the project.


Each need will be linked back to its project and will link to the user requirements created from it. For example:

Projects and Needs

For a full version of this white paper: “Role and Representation of Needs in Systems Engineering Using Cradle” click here